Biomass accumulation and nitrogen fixation by a white clover seed crop: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours in the University of Canterbury

Abstract

The response of seed yield, biomass production and nitrogen fixation of Trifolium repens L. (cv Grasslands Huia) to plant density and nitrogen fertiliser was investigated in a commercial white clover field on the Lincoln College mixed cropping farm, Canterbury. Nitrogen fixation was estimated using a partial nitrogen balance method. Barley (Hordeum sativum) was sown to compare the effect of this crop and white clover on the yield of ryegrass (Lolium multiflorum cv Grasslands Moata) grown in a glasshouse. Greater partitioning of biomass dry matter into inflorescence and petiole in 30 cm row spaced. clover contributed to the higher seed yield of this treatment (632 kg ha⁻¹ ) compared to dense clover (449 kg ha⁻¹ ). Dry matter accumulation was not limited by nitrogen supply with little response to nitrogen fertiliser (75 kg N ha⁻¹ ) being obtained. The mean quantity of nitrogen fixed by a white clover seed crop was 207 kg N ha⁻¹. Nitrogen returned to the soil in clover residues forms a substantial proportion (65 per cent) of the total nitrogen fixed by this crop. Nitrogen fixation decreased by 57 kg N ha⁻¹ as the availability of soil mineral nitrogen increases. Greater availability of soil mineral nitrogen under white clover increased ryegrass yields compared to barley and fallow (4 750, 2 889 and 2 996 kg DM ha⁻¹ respectively over two cuts).